I. Technical Field
The present invention relates to an objective lens driving device, a control circuit, an optical disc device, and an objective lens driving method used to record and/or reproduce information in/from a BD (Blu-ray Disc), a DVD, a CD, an MD (MiniDisc), and so forth.
II. Description of the Related Art
An optical disc device using a disc-shaped optical recording medium (hereinafter, referred to as the optical disc), such as an optical disc and a magneto optical disc, is equipped with an optical head device that reproduces an information signal recorded in the optical disc or records an information signal therein. The optical head device includes a semiconductor laser serving as a light source that emits a beam of light irradiated onto the information recording surface of an optical disc, a beam splitter that separates return light from the optical disc, an optical system block formed of a hologram element or the like, and an objective lens driving device that focuses a beam of light emitted from the semiconductor laser on the information recording surface of the optical disc by means of the objective lens while causing the beam of light to track the information track.
The objective lens driving device is a device that adjusts the objective lens by moving the objective lens in a direction perpendicular to the information recording surface, that is, the focus direction, and in a direction parallel to the information recording surface and orthogonal to the information track of the optical disc, that is, the tracking direction, the direction same as the direction of the radius of the optical disc, for the objective lens to follow the surface deviation and the decentering of the optical disc accompanying rotations of the optical disc.
In order to move and adjust the objective lens in a range of 2 mm in the focus direction and in a range of 1 mm in the tracking direction, the objective lens driving device includes, for example, an orthogonal biaxial actuator mechanism that uses an electromagnetic force induced by a control current fed to a coil disposed in the magnetic field.
As the orthogonal biaxial actuator mechanism in practical use, there have been proposed those adopting the spring supporting structure method by which no friction is caused and a smooth driving characteristic can be obtained and the shaft-sliding structure method by which the assembly accuracy can be readily achieved and an excellent objective lens tilting posture maintaining characteristic is obtained.
In the orthogonal biaxial mechanism adopting the spring supporting structure method, a hinge type structure, a wire type structure, or a plate spring type structure has been known as the structure of elastic supporting members that hold the objective lens. The orthogonal biaxial actuator mechanism of the plate spring type structure is quite useful for a size reduction of the objective lens driving device owing to its workability and operation property.
The conventional objective lens driving device equipped with an actuator portion serving as the orthogonal biaxial actuator mechanism of the plate spring type structure as above is described, for example, in U.S. Pat. No. 3,125,105.
Hereinafter, the conventional objective lens driving device will be described using FIG. 17 through FIG. 19. FIG. 17 is an overall perspective view showing an example of the actuator portion in the conventional objective lens driving device. FIG. 18 is a view showing the configuration of a control circuit in the conventional objective lens driving device. FIG. 19 is a partial sectional side view of the actuator portion shown in FIG. 17.
Referring to FIG. 17 through FIG. 19, an objective lens 1 is made by means of glass press or resin molding, and an objective lens holder 2 is made by means of resin molding and has a hole into which the objective lens 1 is inserted. The objective lens 1 is fixed to the objective lens holder 2 by means of bonding. Also, a focus coil 3 with turns about the Z-axis and tracking coils 4A through 4D with turns about the X-axis are bonded and fixed to the objective lens holder 2. The objective lens 1, the objective lens holder 2, the focus coil 3, and the tracking coils 4A through 4D together form a movable portion 5.
Elastic supporting members 6A through 6D are formed of thin plate spring materials, and one end of each is fixed to the side surface of the objective lens holder 2 and the other end is fixedly bonded to a fixing member 7 made by means of resin molding. The fixing member 7 is fixed to a base member 8. Owing to this supporting structure, the elastic supporting members 6A through 6D support the movable portion 5 so as to be able to oscillate in the focus direction (Z-axis direction) and in the tracking direction (Y-axis direction).
The elastic supporting members 6A through 6D are made by punching a metal plate, which is made of phosphor bronze, beryllium copper, or the like excellent in both the current passing property and the spring characteristic, by means of sheet metal press working, and they also pass a current to the focus coil 3 and the tracking coils 4A through 4D.
The base member 8 is made of ferromagnetic metal, such as iron, and includes a yoke 9A and a yoke 9B that oppose each other with the focus coil 3 and the tracking coils 4A through 4D in between. A permanent magnet 10A and a permanent magnet 10B each having the magnetic pole direction in the X-axis direction and different magnetic poles in the surfaces opposing each other are bonded and fixed to the yoke 9A and the yoke 9B, respectively, thereby forming a magnetic circuit.
A control circuit to drive an actuator portion 901 configured as described above is formed of, as is shown in FIG. 18, a focus control circuit 31 that supplies the focus coil 3 with a current corresponding to a focus driving signal and a tracking control circuit 41 that supplies the tracking coils 4A through 4D with a current corresponding to a tracking driving signal.
In the actuator portion 901 and the control circuit configured as above, when a current corresponding to the focus driving signal is supplied to the focus coil 3 from the focus control circuit 31, an electromagnetic driving force that drives the movable portion 5 in the focus direction is generated by the current flowing through the focus coil 3 and magnetic fluxes from the permanent magnet 10A and the permanent magnet 10B forming the magnetic circuit.
The electromagnetic driving force causes the objective lens 1 to move in the focus direction parallel to the optical axis, so that a focus adjustment operation is performed for a beam of light emitted from the semiconductor laser to be irradiate onto an optical disc. When the focus adjustment operation is performed, the movable portion 5, more specifically, the position of the objective lens 1, is adjusted in the Z-axis direction as the elastic supporting members 6A through 6D whose end portions are fixed to the fixing member 7 undergo elastic displacement in the focus direction (Z-axis direction) in FIG. 17.
Also, in the actuator portion 901 and the control circuit, when a current corresponding to the tracking driving signal is supplied to the tracking coils 4A through 4D from the tracking control circuit 41, a magnetic driving force that drives the movable portion 5 in the tracking direction (Y-axis direction) is generated by the current flowing through the tracking coils 4A through 4D in portions parallel to the optical axis of the objective lens 1 and magnetic fluxes from the permanent magnet 10A and the permanent magnet 10B forming the magnetic circuit.
The magnetic driving force causes the objective lens 1 to move in the tracking direction orthogonal to the optical axis, so that a tracking adjustment operation is performed for a beam of light emitted from the semiconductor laser to be irradiated onto an optical disc. When the tracking adjustment operation is performed, the movable portion 5, more specifically, the position of the objective lens 1, is adjusted in the Y-axis direction as the elastic supporting members 6A through 6D whose end portions are fixed to the fixing member 7 undergo elastic displacement in the tracking direction (Y-axis direction) in FIG. 17.
In order to prevent the occurrence of tilting of the movable portion 5 with respect to the XY plane during the displacement operation of the movable portion 5 in the focus direction and the tracking direction, it is necessary to make the center of gravity of the movable portion 5, the supporting center of the elastic supporting members 6A through 6D supporting the movable portion 5, and the centers of the driving forces generated in the focus coil 3 and the tracking coils 4A through 4D coincide one with another within a plane orthogonal to the direction along which the displacement operation takes place.
Herein, because the objective lens 1 is disposed in the movable portion 5 on the optical disc side, the center of gravity of the movable portion 5 is on the plus side of the Z-axis, and in particular, it coincides neither with the supporting center of the elastic supporting members 6A through 6D nor with the center of the driving force of the tracking coils 4A through 4D. Hence, as is shown in FIG. 19, the center of gravity of the movable portion 5, the supporting center of the elastic supporting members 6A through 6D, the center of the focus driving force, and the center of the tracking driving force are made to coincide with a point CG by lowering the center of gravity of the movable portion 5 with the use of a balancer 90 made of brass or the like and fixedly bonded to the bottom of the objective lens holder 2 (on the minus side of the Z-axis) by means of bonding or fusion bonding.
However, with the conventional configuration as described above, there is a need to additionally provide the balancer 90, which consequently increases the weight of the movable portion 5. This raises a problem that the driving current is increased and so is the power consumption. In addition, because a space in which to attach the balancer 90 is required, there is another problem that it is difficult to make the actuator portion 901 thinner.